1. Field of the Disclosure
The present disclosure relates to a method of manufacturing the blades of a rotor for a gas turbine engine. In particular, but not exclusively, this disclosure can relate to a method of grinding the tips of the blades.
2. Description of the Related Art
With reference to FIG. 1, a ducted fan gas turbine engine is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate-pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate-pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high-pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
It is well known that to maintain an efficient gas turbine engine the gap between compressor blade tips and the engine casing is closely controlled to minimise the leakage of compressed air over the blade tips and back upstream. To this end, the engine casing often includes an abradable liner which provides a close fitting seal with the blade tips.
The abradable liner is initially installed so as to be in contact with the compressor blade tips. During the first few rotations of the compressor rotors, the abradable liner is scored by the rotating fan and compressor blade tips which remove just enough material to allow a free rotation of the compressor blades whilst maintaining a close gap. The abradable nature of the liner allows it to be sculpted by the blade tips to provide a tailored and close fit.
During engine use, the radial positions of the rotating blade tips move due to thermal expansion and vibration. This movement further rubs the abradable liner such that the mean operating gap between the blade tips and liner increases over time. This increases the leakage of air back up the compressor, thereby reducing efficiency and performance of the engine.
FIG. 2 shows a typical but exaggerated profile of an abradable liner 210 caused by compressor blade tip rub over time. The liner includes an outer surface 212 which is attached to the engine casing and an inner surface 214 which faces the rotor. In use, the blade rotates about the rotor axis, travelling in a direction perpendicular to the page, so as to contact and rub the liner. The profile of the rub 218 can be generally described as “M” shaped where the extent of the rub is greater towards the edges 220 of the rotational path of the blades than in the mid portion 222. Although the exact profile of the rub will change between rotors and engines, the “M” shaped profile is a reasonably common occurrence.
During manufacture of the rotors, the blades undergo a grinding process in which a blade assembly is rotated such that the tips of the blades pass a rotating grinding wheel which removes a portion of the blade. Typically, approximately one to two millimetres is removed from each blade tip with the grinding controlled such that the peak to peak height difference between leading and trailing edges of blade tip is typically less than 0.1 mm. In this way, the positions of the each of the blade tips can be controlled during rotation such that the erosion of the abradable liner is reduced. The blades can also undergo other processing operations during manufacture, such as measurement validation of rotor concentricity.
There are two main types of compressor blade root construction:                Circumferential, where the roots of the blade are loaded one after another into the rotor via a circumferential slot and moved round circumferentially until the stage is full, and        Axial, where the roots of the blades are loaded into individual axially directed slots which are generally at an angle to the engine centreline.        
FIG. 3 shows an axial root blade 250 on the left and circumferential root blade 252 on the right.
In FIG. 4 there is shown a cross section of an intermediate compressor rotor 310 having a blade 312 with a circumferential root 314 which is snugly received within a slot 316 in a disc 318. The blade includes a tip 320, a leading edge 322 and a trailing edge 324.
There is a degree of movement provided between the blade root 314 and disc slot 316 in order for the blades to be slotted into place. This allows the blade 312 to rest in different positions within the rotor as shown by the positions indicated by reference numeral 326a, the second by 326b and the third by 326c. The second position 326b represents the mid-point of the blade 312 within the slot 316 with the other two positions demonstrating the range of movement.
During use, the rotor is rotated at several thousand rpm which results in a radial centrifugal force acting on the blade 312. This results in a stiction between the shoulder of the root 328 and corresponding opposing surface of the slot 316, which keeps the blade 312 in a fixed position. It has been previously known that blades can move when the rotor 310 slows to a halt and the centrifugal force that locks the blades in place no longer applies. Thus, when in service, the blade 312 may fall into one or other of the extreme positions where they will remain until the engine is next started.
The above sequence of events leads to the movement of the blade and the different tilt positions leads to the tips of the blades being higher or lower in relation to the abradable liner. This results in the “M” shaped profiled in the liner. However, the extent of the “M” profile is greater than could be explained by the movement of the blades in this way.
In particular, it is known that the blades held within a blade assembly for grinding can move prior to the grinding operation, particularly when the grinding process is stopped part way through and restarted as is sometimes necessary if a blade is damaged and needs replacing. If one or more of the blades does move during this process, for example, from position 326a to position 326c as shown in FIG. 4, then the profile created by the grinding process is effectively skewed.
If the position of a blade alters during the grinding process then they will have different profiles with respect to one another. FIG. 5(a) shows a pair of blades 410, 412 which have moved during the grinding process and are no longer aligned. Although the grinding of the blade tips is well controlled such that the tip variance between blades is less than 100 microns, the different positions of the blades 410, 412 are not accounted for. Hence, as shown in FIG. 5(b), if the blades 410, 412 move relative to the disc during service of the engine e.g. when the rotor comes to rest after a period of use, the blades 410, 412 can swap tilt positions and the error which would ordinarily be expected is doubled. The actual difference this leads to can be in the order of hundreds of microns.
Although discussed above in relation to circumferential root blades, similar problems of blade movement and different forward/rearward tilt positions can apply to axial root blades because of the angling of their roots relative to the engine centreline.
For example, FIG. 6 shows schematically a rearwards directed view of blade tip of a blade with an axial root for different levels of tilt. If the blade is tilted fully anti-clockwise (as shown by the blade to the left in FIG. 6) in the root when it is ground the blade is X1 longer than a blade that was centralised (shown by the central blade in FIG. 6) during grind. If this blade then rotates fully clockwise (as shown by the blade to the right in FIG. 6) during a subsequent run in the engine, the blade tip is (X1+X2) radially further outward that the nominal desired tip position. This may be a significant amount and results in the blade tip rubbing out the abradable liner by an additional (X1+X2) depth. This increases running clearances by (X1+X2) for a blade with a nominal blade tip position. For a blade that was tilted clockwise during grinding but then tilts anticlockwise during running, the blade tip will be (X1+X2) radially more inward than a nominal tip. This means if this blade is running within a casing that has suffered the (X1+X2) additional rub an equivalent additional tip clearance will exist. Accordingly, the effect on running clearances and hence compressor efficiency and surge margin may be significant.
Another unfortunate aspect of blade tilt associated with axial roots, is the effect it has on the angle of the tip of the longest blade. In the example described above and shown in FIG. 6, the drum rotational direction is clockwise i.e. from left to right in the diagram. The longest blade rotates clockwise by φ° relative to the position it was in when it was ground. This results in the blade tip having a negative relief (or clearance) angle with the casing of φ°, with the suction surface edge touching the casing rather than the pressure surface edge. This negative relief angle makes the blade tip an inefficient cutting tool, such that for high incursions significant heat and blade vibration is created. This can result in over-cutting of the soft abradable liner due to heat build-up, softening of the liner material, and its deposit on the longest blade doing the rubbing. It can also result in aerofoil cracking due to vibration.
Excess blade tilt can also be a problem in other processing operations, such as when measuring blade stage concentricity, because variation in the blade tip positions around the circumference can cause false readings and bias in blade location analysis.
US 2012/202405 proposes a method of grinding blade tips which aligns the blades in a predetermined position using compressed air before the grinding operation.